The tropical warble fly or torsalo, one of the most important parasites of cattle in Latin America, is distributed between southern Mexico and northern Argentina. Larval stages are found in many hosts, including cattle, sheep, goats, pigs, buffalo, dogs, cats, rabbits, and people. Cattle and dogs are infected most commonly. D hominis is thought to initiate the lesion that gives rise to lechiguana, a disease of cattle (see Lechiguana).
The adult fly is 12–15 mm long and has a short life span (1–9 days). The adult fly fastens its eggs to different types of insects (49 have been described as vectors of D hominis in Latin America; most are mosquitoes or muscoid flies) that then transport them to warm-blooded hosts, where they hatch as the insects feed. The larvae penetrate the skin of the animal within a few minutes of hatching and remain in the subcutaneous tissue for 4–18 wk. During this period, the larvae grow within warbles with breathing holes. When mature, the larvae leave the host and drop to the ground, burrow, and pupate. After the pupal period, which lasts 4–11 wk, the flies emerge as adults. The complete life cycle takes 11–17 wk.
Clinical Findings and Pathogenesis
Larval penetration of the skin is accompanied by pain and local inflammation, and pus gradually forms. Infested hides are condemned at slaughter, and production of milk and meat is reduced. In 1982, it was estimated that cattle infested with D hominis resulted in a yearly reduction in weight of 40.6 g/larva. In 2014, combining losses due to reduced weight gain and hide damage, infestation by D hominis in cattle raised in Brazil was estimated to cost producers $383 million USD annually. Efforts to characterize and produce antigenic proteins that confer immunity against D hominis have resulted in identification of a candidate vaccine, with 90% efficacy in immunized cattle.
Treatment and Control
Different contact and systemic insecticides in various formulations are available for treatment. Generally, D hominis infestations in cattle are susceptible to systemic organophosphates and macrocyclic lactone endectocides, which may be approved and available locally.
Lechiguana is a sporadic, chronic disease of cattle that, thus far, has been reported only from southern and southeastern Brazil, in areas where infection by Dermatobia hominis is common. It is characterized by large, hard, subcutaneous swellings that develop rapidly, mainly in the scapular and adjacent areas (chest, neck, shoulders, and ribs). Most cattle affected have only one swelling, but two swellings are occasionally seen. The regional lymph nodes are enlarged and without treatment may become enormous.
Mannheimia granulomatis has been recovered from lesions and is considered causal. The lesion that gives rise to lechiguana is initiated by D hominis larvae. M granulomatis is consistently recovered from lesions of the clinical disease, and it is thought to be mainly responsible for the characteristic tissue changes. The involvement of a myofibroblast-like cell population that expresses mRNA for type I collagen is suggested to be associated with the increase of collagen deposition. It has also been demonstrated that macrophages activated by M granulomatis induce fibroblast proliferation. The habitat or source of M granulomatis is not known. It has not been recovered from cattle without lechiguana.
Histologically, lesions consist of focal proliferation of fibrous tissue infiltrated by plasma cells, eosinophils, lymphocytes, and sometimes neutrophils. The primary lesion is an eosinophilic lymphangitis, which results in eosinophilic abscesses, with occasional rosettes containing bacteria in their centers. The subcutaneous, tumorous mass produced may attain a size as large as 40 × 50 cm in 2 mo. Without treatment, death occurs after 3–11 mo, probably due to inanition resulting from the enormous swellings.
When well established, the disease is clinically obvious. Diagnosis is confirmed by recovery of M granulomatis and observation of the characteristic histopathologic lesions.
Treatment with chloramphenicol (3 g/day for 5 days) or danofloxacin mesylate (1.25 mg/kg/day for 3 days) results in rapid reduction of swellings, with almost complete regression in 30 days. Conducting susceptibility tests is advisable before using other antimicrobials.
Last full review/revision March 2015 by Mason V. Reichard, PhD; Franklin Riet-Correa, PhD